If they are organising the wedding I'm sure they will cope well. If you can, just sit back and let them get on with the work! The prerogative of parents of grown up children I like to think.
They were already aware of your PD and how you and your wife felt about the occasion so must have thought about it. I'm sure they would not want you to feel you had to stay out of the way in your own home.
I do know what you mean about them not fully understanding how you are now, I'm sure my children don't. It's not that they don't care. Just that Mum's always been around and still looks pretty much the same so......... They're busy with their own lives. I too get easily irritated, often when I'm tired. My husband is very understanding but I still feel guilty for sometimes being short with him.
Both our son and daughter had DIY weddings, which they organised themselves, (admittedly before I was PD diagnosed) and they were lovely occasions, much more personal and memorable than ones organised by a "professional".
Hope you and your family have a wonderful day and make lots of lovely memories. Enjoy you place as father of the bride.